For measuring electrically conductive fluids, flowmeters utilizing a magneto-inductive measurement pickup are often employed. As is known, especially also the volume flow rate of electrically conducting fluids, especially liquids, flowing in a pipeline, can be measured and the measurements reflected in corresponding, measured values. The measurement principle of magneto-inductive flowmeters rests, as is known, on the fact that an electric voltage is induced due to charge separations in a volume fraction of a flowing fluid traversed by a magnetic field. The voltage is sensed by means of at least two measuring electrodes and further processed in a measuring device electronics of the flowmeter to a corresponding, measured value, for example a measured value of a volume flow rate. Equally known to those skilled in the art is the construction of the individual components and the inner workings of magneto-inductive flowmeters. Examples of this technology are contained in DE-A 43 26 991, EP-A 1 460 394, EP-A 1 275 940, EP-A 12 73 892, EP-A 1 273 891, EP-A 814 324, EP-A 770 855, EP-A 521 169, U.S. Pat. No. 6,763,729, U.S. Pat. No. 6,658,720, U.S. Pat. No. 6,634,238, U.S. Pat. No. 6,595,069, U.S. Pat. No. 6,031,740, U.S. Pat. No. 5,664,315, U.S. Pat. No. 5,646,353, U.S. Pat. No. 5,540,103, U.S. Pat. No. 5,487,310, U.S. Pat. No. 5,210,496, U.S. Pat. No. 4,704,908, U.S. Pat. No. 4,410,926, US-A 2002/0117009, or WO-A 01/90702.
For conveying the fluid being measured, measurement pickups of the described kind exhibit, as also shown schematically in the appended figures, a measuring tube inserted into the course of the pipeline conveying the fluid. For preventing short-circuiting of the voltage induced in the fluid, the measuring tube is embodied essentially electrically non-conductively, at least on its inside contacting the fluid. For the inserting of the measuring tube into the course of the pipeline conveying the fluid, the ends of the measuring tube are provided with flanges or the like. Measurement pickups of the described kind, as used industrially, have, in such case, most often, a measuring tube built by means of a metal support tube and a coating—the so-called liner—of an electrically insulating material internally applied thereto. The use of a measuring tube built in this way assures, among other things, a mechanically very stable and robust construction of the measurement pickup and thus also of the flowmeter as a whole. As material for the liner, materials such as e.g. hard rubber, polyfluoroethylene, polyurethane or other chemically and/or mechanically durable plastics are used, while the support tubes of the described kind, in order to prevent a degrading of the magnetic field, especially also a possible short circuiting of the same over the measuring tube, are conventionally manufactured of a non-ferromagnetic, especially paramagnetic, material, such as e.g. stainless steel or the like. Thus, by an appropriate selection of the support tube, a matching of the strength of the measuring tube to the mechanical demands present in the particular case of use can be realized, while, by means of the liner, a matching of the measuring tube to the chemical, especially hygienic, demands existing for the particular case of use can be obtained. Usually, in such case, materials are used, which have a nominal, thus effective, or average, relative permeability μr, which is essentially smaller than 10, especially smaller than 5. As is known, relative permeability μr measures, in such case, how much the magnetic flux density (=magnetic induction) is increased relative to the magnetic flux density in air or vacuum, whose permeability μ0 (=induction constant) is, as is known, equal to 1.256·10−6 Vs·Am−1, when the material of concern is placed in the same magnetic field, i.e. the permeability μ of the material being used equals μr·μ0.
The magnetic field required for the measurement is produced by a corresponding magnetic field system composed of a coil arrangement, including, most often, two field coils, corresponding coil cores and/or pole shoes for the field coils and, as required, magnetically conductive, field-guiding sheets connecting the coil cores outside of the measuring tube. However, there are also magnetic field systems known using only a single field coil. The magnetic field system is usually, as, in fact, indicated in FIG. 1, arranged directly on the measuring tube and held by such.
For producing the magnetic field, an exciter current I delivered by a corresponding measuring device electronics is caused to flow in the coil arrangement. The exciter current is, in the case of modern measurement pickups, usually a pulsed, bi-polar, rectangular, alternating current. U.S. Pat. No. 6,763,729, U.S. Pat. No. 6,031,740, U.S. Pat. No. 4,410,926, or EP-A 1 460 394 give examples of circuit arrangements serving to produce such exciter currents, as well as corresponding switching and/or control methods therefor. Such a circuit arrangement includes, usually, an energy, or power, supply driving the coil current, as well as a bridge circuit, in the form of an H-, or T-, circuit, for modulating the exciter current.
The voltage, generated in the fluid according to Faraday's law of induction, is sensed (to provide the measured voltage) between at least two galvanic (thus wetted by the liquid), or at least two capacitive (thus e.g. arranged within the tube wall of the measuring tube), measuring electrodes. In the most common case, the measuring electrodes are so arranged diametrally opposite one another that their shared diameter is perpendicular to the direction of the magnetic field and thus perpendicular to the diameter, on which the coil arrangements lie; equally as well, the measuring electrodes can, however, also be arranged non-diametrally opposite one another on the measuring tube; compare, in this connection, especially U.S. Pat. No. 5,646,353. The measured voltage sensed by means of the measuring electrodes is amplified and conditioned by means of an evaluating circuit to provide a measurement signal, which can be recorded, displayed or even further processed. Corresponding measuring electronics are likewise known to those skilled in the art, for example from EP-A 814 324, EP-A 521 169, or WO-A 01/90702.
As already indicated, for measurement pickups of the described kind, guidance of the magnetic field within and outside of the measuring tube has a special importance. Usually applied measures for influencing the magnetic field include, along with the use of non-ferromagnetic measuring tubes, for example, as described among other places also in U.S. Pat. No. 6,595,069, the use of suitably formed pole shoes arranged for the field coils as close as possible to the fluid and/or the use of magnetically conductive, especially ferromagnetic, materials for the guideback of the magnetic field outside of the measuring tube.
A significant disadvantage of such measurement pickups with metal support tube is to be seen in the fact that, on the one hand, considerable technical skill is required, in order to form and guide the magnetic field in degree sufficient for the required accuracy of measurement. On the other hand, the use, associated therewith, of relatively expensive, non-ferromagnetic metals for the support tubes, such as e.g. paramagnetic stainless steels, represents another significant cost factor in the manufacture of measurement pickups of the described kind. A further disadvantage of conventional magnetic field systems is that the magnetic field is, as schematically pictured in FIG. 1, very inhomogeneously developed inside of the measuring tube lumen and, therefore, the measured voltage can depend in significant measure also on the flow profile of the fluid in the measuring tube.